China and the US are the two elephants in the climate change room, the two economic powers that between them account for half of the world's annual emissions of greenhouse gases. Both have been judged major obstacles to the global effort to reduce emissions, and each has used the other as its prime excuse: US conservatives argue that the US should only reduce emissions once China and other major developing countries have also pledged to do so. China insists that the problem was created by rich countries and they have the means and the responsibility to fix the problem.

On the surface these attitudes seem to spell deadlock. But behind the public posturing a different process is unfolding, triggered by the departure of George Bush from the White House and China's ambitions to increase the manufacture of high-value technologies. The US is back in the global climate game, even, belatedly, claiming leadership. China is obliged to respond, or risk bearing all the opprobrium.

Today, the world's biggest emitters are led by governments that understand the climate threat and their own exposure to future risks and international hostility. Each has the power to wreck agreement on a deal, but together they have a huge potential to drive the real reductions in emissions that the world needs. As the Copenhagen deadline approaches, all eyes are fixed on the possibility of a bilateral deal that could transform the future.

With Obama's election, the way was opened for the US and China to look for the mutual benefits of cooperation on climate, instead of clinging to mutually assured destruction. The question now is whether thesynergies offered by China's potential for large-scale manufacturing and deployment and American technological innovation can overcome the rivalry between the incumbent superpower and its biggest economic rival. At the end of a two-day visit by Obama's chief climate negotiator, Todd Stern, the mood music in Beijing this week was promising.

Both have much to gain. The Chinese leadership is well aware of the dangers that climate change poses to its future development. China is highly vulnerable to sea level rise and to falls in agricultural production. Above all, it is haunted by the water shortage which already grips north China and which threatens to become Beijing's most intractable crisis as the melting glaciers of the Himalayas wreak havoc with Asia's rivers and monsoon patterns.

Beijing is looking for the chance to move beyond the exhausted and wasteful model of export-led manufacturing and into high-value technologies. The rewards of becoming a leader in the clean, low-carbon technologies of the future are perhaps better understood in Beijing than in some western capitals. As GM collapses, strangled by its attachment to old models, China is developing electric cars.

In this fast moving game, both powers know that the chances of striking a global deal in Copenhagen in December that will prove sufficiently radical are slim. A new climate treaty remains vital, but a treaty alone will not produce the radical emissions reductions the crisis demands. Both risk being seen as selfish powers indifferent to the threat climate change poses to humanity. Each faces substantial domestic opposition: the US labours to discard the legacy of eight wasted years under Bush. Today, despite his claim of climate leadership, Obama's ambitions remain constrained by a recalcitrant domestic opposition. Beijing has to negotiate with powerful regional leaders who need to deliver jobs and growth at home, Obama with powerful oil and coal interests stirred into action by the change in the US position.

But despite China's ambitions to continue its rapid growth, Beijing knows that the carbon-heavy path it is following would render the efforts of the rest of the world to reduce global emissions futile. A leading economist, Hu Angang, recently proposed, in a radical article, that China should take on early emissions reduction targets for its own interests. For the two big emitters, the prize is to find a bilateral deal that enables them to share profits rather than sacrifice them, and that promise has to be convincing enough to overcome the doubts of domestic constituencies that see the other as the main strategic rival.

China is already the world's leading investor in renewable technologies and government encouragement has produced a surge in installed wind power. Now that China's target of 15% renewable energy by 2020 will be upgraded to 20%, China can argue that its efforts to avoid dangerous climate change are at least on a par with the EU, currently the leader in declared targets. Although under the Kyoto treaty China is not obliged to take on reduction targets, Beijing does aim to stabilise emissions by 2020. Given China's soaring energy needs, advanced technology and energy efficiency will be key components of that effort.

China might achieve this alone. In partnership with the US, both have the potential to become the solution rather than the problem.